its difficult to comment as you are altering the dimensions but how about "fettling" U1 & U2 to make them fit correctly on the underside of TR1 & TR2,
then
make your suspension bracket jig as per the instruction in the book, place it into position on the chassis & you,ll then see how much "gap" is created by you altering the dimensions of the chassis, then maybe welding an additional section of box tube to the outer faces of U1 & U2 so that the suspension bracket jig meets the required positioning......just an idea
cheers
andy